This invention pertains to a multistage, extensible-contractible hydraulic ram, and more particularly to a single-acting, three-stage, four-section telescopic ram which is especially adapted for use in conjunction with a triple-lift-type mast in a lift truck.
Many industrial lift trucks are equipped with a so-called triple-lift-type, vertically extensible-contractible mast. Such masts include inner, intermediate and outer sections which are extensible vertically relative to one another, with the inner section carrying a carriage on which any one of a variety of load-handling attachments, such as lift forks, may be mounted. The carriage is disposed for travel up and down on and along the inner mast section during what is normally referred to as the "free-lift" operating stage of the truck. Raising and lower of such a mast is accomplished through mechanism substantially always including a multisection hydraulic ram, with various sheaves and chains used to provide operative interconnections between the ram and mast sections.
A number of features are usually sought in such an arrangement. For example, it is important that during times when a lift truck is being driven and maneuvered from one location to another, the operator have good forward visibility. Under such a driving situation, it is usual that a mast and ram are substantially fully contracted or lowered.
Further, in order to avoid jolting of a load as it is raised and lowered, it is desirable that the raising and lowering speed of a carriage, throughout the entire vertical range of travel of the carriage, be as constant as possible. Jolting will occur, of course, if any abrupt changes in speed take place. Obviously, such jolting presents the serious hazard of dropping a load and causing personal injury and/or other damage.
Additionally, it is desirable that the sequence in which various movable parts in and on a mast travel with respect to one another be controlled positively. In other words, for each type mast and associated carriage there is a specific sequence of related movements which are proper, and it is desirable to insure that nothing causes "mis-sequencing" of these parts.
Still another consideration is that while it is important to obtain good visibility for an operator with the mast and ram contracted, this characteristic should not, if possible, affect a "high-lift" capability for a load.
A general object of the present invention, therefore, is to provide a novel multistage ram which is capable of being incorporated with a triple-lift-type mast in a lift truck, in a manner which will take all of the above-mentioned concerns and considerations into account in a very practical and satisfactory manner.
According to a preferred embodiment of the invention, the ram comprises, basically, a single-acting, four-part, three-stage construction, including a cylinder, an outer large piston slidably mounted within and extensible from the cylinder, an intermediate piston slidably mounted within and extensible from the outer piston, and an inner piston slidably mounted within and extensible from the intermediate piston.
In this arrangement, pressure fluid introduced to extend the ram enters the cylinder, and is applied against the effective working surface area of the outer piston, and also simultaneously against the effective working surfaces of the intermediate and inner pistons. The latter conditon results from the presence of an open fluid passage system extending through the outer piston between the outer cylinder and the intermediate and inner pistons. Provided in this fluid passage system is a valve that is held open with the ram contracted, which valve closes later with extension of the outer piston for the purpose of inhibiting flow between the opposite sides of the latter.
Provided between the cylinder and the side of the outer piston which is opposite that having what was just called its "effective working surface area", is a space containing fluid, which space connects through another fluid passage system with a portion of the first-mentioned fluid passage system. A check valve disposed within this other passage system blocks the escape of fluid from the space with the ram contracted. Such blockage prevents initial movement of the outer piston. An actuator for opening this valve is included, which actuator operates to open the valve at a certain condition of ram extension, which condition will be explained shortly.
Ideally, the effective working surface area on the outer piston is twice that on the intermediate piston. Also, the dimensions of the space just mentioned are such that the volume of fluid which escapes this space with extension of the outer piston at a certain speed relative to the cylinder, is exactly the same as the volume of fluid required to produce extension of the inner piston at the same speed relative to the intermediate piston.
With pressure fluid supplied to the cylinder at a substantially constant rate, what first occurs is that the intermediate piston extends at a certain speed relative to the outer piston. The inner piston is carried as a unit with the intermediate piston. The outer piston remains stationary because of the fact that the escape of fluid from the "space" mentioned is prevented.
Upon the intermediate piston reaching substantially full extension, a portion thereof engages and acts upon a stem connected to the actuator of the second-mentioned valve -- opening this valve. Such opening permits the escape of fluid from the space between the outer piston and the cylinder, and hence, permits movement of the former.
Continued supplying of pressure fluid at the same rate then causes extension of the outer piston at half the speed at which the intermediate piston previously extended. Such movement of the outer piston results in the first-mentioned valve closing to block the flow of fluid through the outer piston. As a consequence, fluid escaping from the "space" now flows into the hollow interior of the intermediate piston to act on and cause extension of the inner piston relative to the intermediate piston. This action continues until full extension of the ram. The speed at which the inner piston extends relative to the intermediate piston is the same as that at which the outer piston extends relative to the cylinder. Hence, the overall extension speed of the ram is substantially constant from full contraction to full extension.
Positive sequencing is achieved through the use of the two valves discussed, as well as through the selection of the specific relationships mentioned between the effective working surface areas on the pistons.
Thus, the ram of the invention can readily be incorporated with a triple-lift-type mast to produce the desired kind of performance mentioned above. The fact that the ram includes four sections insures its having a relatively short overall length when fully contracted, which condition contributes to good forward visibility for a truck operator.